This project involves a study of the mechanism of selective antibody absorption by the human placenta and guinea pig yolk sac placenta. The methods used for study include localization of antibodies in the tissue by immunocytochemical techniques, or the absorption, in vitro, of enzyme-conjugated antibodies. The localization of the molecules and their pathways of transport are examined by light and electron microscopy. A comparison will be made between the localization of the various classes of immunoglobulins in the placenta. By similar methods, the localization of enzymes involved in immunoglobulin catabolism in the placenta or yolk sac is being examined. The fine structure, permeability, protein absorptive activity and differentiation of the amnion will be examined utilizing electron microscopy and electron-dense tracers.